The present invention relates to automobile wheel alignment systems In particular, the invention relates to a method and apparatus for measuring a cross-toe angle in a wheel alignment system.
Proper alignment of wheels in an automotive vehicle is important for proper handling of the vehicle and proper tire wear. In addition, the proper alignment of the wheels will decrease fuel consumption as well as increase vehicle safety. The wheel alignment parameters which are measured and adjusted in order to achieve proper wheel alignment are camber, caster, steering axis inclination and toe.
The toe angle of a wheel, at a specified wheel load or relative position of the wheel center with respect to a sprung mass, is the angle between a longitudinal axis and a line of intersection of wheel plane and road surface. The wheel is "toed-in" if the forward portion of the wheel is turned toward a central longitudinal axis of the vehicle, and "toed-out" if turned away. Accurate toe adjustment cannot be attained without properly calibrated measuring instruments.
Camber is defined as the angle which the wheel makes with respect to a vertical plane when looking at the wheel from either the front or the rear of the automobile. Any wheel of an automobile can have camber.
Caster is an angle which the steering axis of a steerable wheel (typically one of the front wheels) makes with respect to the vertical plane which is perpendicular to the longitudinal direction of the automobile.
Steering axis inclination (SAI) is the inclination of the steering axis with respect to a vertical plane which is parallel to the longitudinal direction of the automobile.
Toe was originally defined as the difference of the distance between the front edges of two opposite wheels and the distance between the rear edges of the same two wheels. Toe can also be defined in terms of an angular relationship of the plane of a wheel (perpendicular to its turning axis) relative to a predetermined longitudinal vehicle axis. When the forwardmost portions of a pair of adjacent wheels are closer together than the rear portions of those wheels, they are commonly referred to as in a "toe-in" condition. When the rear portions of wheels are closer together than the front portions, the condition is commonly referred to as "toe-out."
Camber is a tire wearing angle and also affects handling of the automobile. Caster and SAI do not affect tire wear, but are important for directional control. Toe is a tire wearing angle and also affects the position of the steering wheel required in order to maintain the automobile along a straight line.
Wheel alignment in an automobile wheel alignment system can be measured with angle sensors which use a light source and a light sensor mounted on head units which are coupled to the wheels of the automobile. The light source is aimed at the sensor and the sensor provides an output which represents an angular relationship between the sensor and the light source.
In measuring toe, however, if the light source or the sensor is tilted out of a horizontal plane, the tilt introduces an error into cross-toe angle measurements. (Cross-toe angle measurement refers to the toe angle measurement of an individual wheel).
It is desirable to lower the head units so that light from the light source is not blocked by obstructions such as front end air spoilers which may be present on, for example, a sports car. To clear an obstruction, typically a "drop block" is used by an operator of the automobile wheel alignment system. Drop blocks are used to couple the head units to the front wheels of the automobile under test. Drop blocks allow the head unit to be lowered in a vertical plane where an axis of the head unit remains parallel, although no longer co-axial, with an axis of the wheel. However, a minor deformity in the drop block arising during manufacturing or field use can introduce an error into alignment measurements. The drop blocks are also cumbersome and time consuming for the system operator to use.
Cross-toe angle sensors must also be leveled in a horizontal plane to obtain accurate cross-toe alignment measurements. Both with and without the use of drop blocks, the system operator must precisely align the head units in the horizontal plane. This is a source of errors in the alignment measurements, and is also time consuming for the operator.
There is continuing need for improved angle measurement systems which are easy for an operator to use and which provide more accurate measurements.